There are two categories of stylus pens, passive pens, such as capacitive pens, and active pens, such as ultrasound pens and electromagnetic resonance (EMR) pens. There are various situations that require palm rejection (i.e., distinguishing between a palm touch and a pen or finger touch) while a user is using a stylus pen. For example, ultrasound pens can be used to write both on screen and off screen. For on screen usage, a user's palm often rests on the touchscreen while writing. Such a palm touch should be rejected/ignored, but the high level operating system (HLOS) may not be able to distinguish between the palm touch and a finger touch. As another example, while writing, the user may lift the pen up momentarily and use a finger for gesture control, such as a pan or zoom, and then start writing again. In these scenarios, it would be beneficial for the touchscreen to show the pen input without any palm induced traces. It would also be beneficial for the user to be able to use a finger to perform touch controls on screen when not writing.
Current solutions are not sufficient to meet these requirements. One solution disables finger touch detection when the pen is within two to three inches of the touchscreen. In normal usage, however, the user's palm can be resting on the device even with the pen two or three inches above the device. Further, the accuracy of determining the stylus pen to be within a certain zone above the touchscreen is dependent on the technology. For example, accurate ultrasound pen proximity detection may be more challenging than that of an EMR-type stylus pen that has a more uniform inductive grid under the touchscreen.
Further, the touchscreen should remain active for gestures. Various current solutions use complex algorithms to distinguish traces generated by a finger while ignoring palm touch traces. The results, however, can be inconsistent depending on the size, orientation, or relative movement of the user's palm.
Accordingly, current solutions fail to perform palm rejection effectively in at least the following scenarios: (1) the user wishes to start writing on the touchscreen, but before the pen is hovering over or touches the touchscreen, the user's palm/wrist is already resting on the touchscreen, causing palm induced traces on the touchscreen; (2) the user pauses writing for a moment and uses a finger touch gesture to zoom the content, but the pen is not lifted high enough to get out of the sensing zone, so the gesture input is ignored; (3) like (2), except that the pen is still touching the touchscreen, but not moving.